Hydraulically-actuated fuel injectors utilize a fluid driven piston to pressurize fuel within the injector body. The piston has a pressure surface exposed to fluid pressure in an actuation fluid cavity. An injection event is initiated by switching the actuation fluid cavity from connection to a low pressure drain to a high pressure inlet. The piston is normally biased toward a retracted position by a spring, but begins to move downward when the fluid pressure force overcomes the spring.
Downward movement of the piston intensifies the downward force on a plunger that pressurizes fuel in a fuel pressurization chamber. A biasing spring acting on a needle valve member normally holds the nozzle outlet of the fuel injector in a closed position, but the needle valve member lifts to open the nozzle outlet when the fuel pressure exceeds a predetermined valve opening pressure. Generally, it is desirable that the fuel pressure go from its minimum to its maximum pressure as quickly as possible so that the fuel pressure is as high as possible when the needle valve member in opening. Higher fuel pressures typically result in better atomization of the fuel entering the combustion space. This in turn typically results in more efficient combustion, which tends to result in improved emissions from the engine.
It has been observed that the quality of the initial portion of the injection can be improved if the initial acceleration of the piston can be increased. In other words, if the time taken for the piston to go from zero to its maximum speed is decreased, the speed at which the plunger moves will be increased, and hence the fuel pressure will rise much more rapidly. This problem is complicated by the fact that some finite amount of time is required for the actuation fluid cavity to rise in pressure from that of the low pressure drain up to that of the high pressure inlet.
The present invention is directed to overcoming these and other problems, as well as generally improving the performance of hydraulically-actuated fuel injectors.